Corrosion resistant phosphate coating and method for producing same



United States Patent 3,268,367 CORROSION RESISTANT PHOSPHATE COATING AND METHOD FOR PRODUCING SAME Henry J. Nelson, Grosse Pointe Woods, Mich, assignor to Hooker Chemical Corporation, Niagara Falls, N.Y.,

a corporation of New York No Drawing. Filed Nov. 13, 1962, Ser. No. 237,340

Claims. (Cl. 148-615) The present invention relates to the art of forming phosphate coatings on metallic surfaces and more particularly relates to an improved method of forming zinc phosphate coatings by immersion.

Light weight zinc phosphate coatings are widely employed as a base for paint, lacquer, enamel and other siccative coatings on metal surfaces which are susceptible to corrosion, and heavier coatings are employed as a final finish coating after a chromic acid rinse or oiling or both. Such final finish zinc phosphate coatings are normally produced by immersion processes, are much heavier than paint base coatings and because of their high degree of corrosion resistance, are suitable for use on small parts such as nuts, bolts, interior hardware, and the like. While the corrosion resistance of such heavy phosphate coatings has been considered in the art to be satisfactory for certain applications, the corrosion resistance has been insufiicient for a variety of other types of applications having more severe corrosion conditions during use, for example, uses in which the parts are subjected to high humidity, weathering or salty atmospheres such as occurs to automotive parts or the like.

In accordance with this invention, it has been found that the corrosion resistance of heavy zinc phosphate coatings produced by immersion of the part to be coated in the phosphate solution is substantially improved by modifying a conventionally formulated zinc phosphate coating bath to include certain quantities of certain saturated aliphatic polycanboxylic acids. It has also been found that the corrosion resistance is even further improved by using in conjunction with such saturated aliphatic polycarboxylic acids a limited quantity of certain metallic ions such as Ni, Co, Li, Bi, Ce and Mn.

The saturated aliphatic polycarboxylic acids which are useful for the purposes of this invention are those having 26 carbon atoms and in which the carbon atom adjacent to at "least one of the carboxy groups is substituted with a radical selected from the group consisting of a hydroxy, amino and car-boxy radicals. Examples of acids which fall into this class include citric acid, oxalic, malic acid, glutamic acid, tartaric *acid, aspartic acid and malonic acid. Unusually good results have been obtained from the use of citric acid and because of these results, its ready availability and ease in handling citric constitutes the preferred acid for the purposes of this invention.

Generally stated, the baths of this invention may contain the ingredients set forth below in Formulation I.

FORMULATION I A preferred bath for the purposes of this invention is set forth in Formulation II.

ice

FORMULATION II Composition,

Constituents: percent w./v.

Zinc O.lO.7

Ratio e++ 2 .45 P-olycarboxylic acid .51.5

Metallic ion .O04.006

The baths may have a total acid in the range of 15- points and preferably are operated at a total acid of 2560 points. Points of total acid refer to the number of ccs. of N 10 sodium hydroxide required to titrate a 10 ml. sample of the bath to a phenolphthalein end point.

It has been found that the addition of one of the polycarboxylic acids, as above identified, to a zinc phosphate coating bath of the type defined in Formulation 1 causes a significant and substantial improvement in salt spray corrosion resistance of the resulting coating. It was surprisingly found, however, that when such a citric acid modified bath of the type specified in Formulation I is further modified to include a very small quantity of a metallic ion in the proportions set forth in Formulation II that a further significant increase in salt spray corrosion resistance is obtained and the quantity of the increase in corrosion which results from the use of the combination of one or more of the metallic ions and one or more of the specified polycarboxylic acids is such that the coated part is sufiiciently corrosion resistant to enable use of the coated part for many commercial uses not heretofore considered to be feasible. The combined presence of the polycarboxylic acid and one or more of the specified metallic ions has been observed to substantially increase the coating Weight which is obtained in a given contact period, for example, 30 minutes immersion at F.2l0 F. Under such conditions the coatings which are obtained have a weight of between about 30% to 100% higher than is obtained from an otherwise similar bath except for the presence therein of the polycarboxylic acid and the metallic ion. The best corrosion results have been obtained from the use of combinations including the nickel ion, and a concentra-:

tion of nickel of about 0.004% w./v. has been found to be the best concentration for use. When one of the other metallic ions is employed, in lieu of the nickel ion, the substituted ion is preferably employed in a molar quantity equivalent to about 0.004% nickel.

Baths of this invention may be made to contain the metallic ion concentrations set forth above in Formulation I or II from conventional starting materials such as zinc oxide, phosphoric acid, zinc di-hydrogen phosphate, zinc nitrate, and the like. The metallic ion is suitably provided in the form of nitrate, phosphate, or carbonate salt and the polycarboxylic acid may be added in the form of the acid or one of its salts which ionizes to release the acid in aqueous solution. The ferrous ion concentration is extremely important to the successful formation of the improved corrosion resistant coatings of this invention and is preferably provided in the bath by processing ferrous parts through a bath after it is otherwise completely formulated. It' is possible to incorporate the ferrous ion in the solution in the form of a ferrous salt such as ferrous phosphate but it has been observed that the best corrosion resistance is not obtained in such a solution until the solution has been broken in by processing additional ferrous parts through the bath under operating conditions. The explanation for this phenomenon is not completely understood but it is believed that the polycarboxylic acid and iron combine to form a complex of unknown constituency, which complex thereafter functions in the solution and modifies the formation of the coating on the metal surface being treated.

As above indicated in Formulation I, the operating bath preferably contains the nitrate ion, the concentration of which may be varied substantially but it has been observed that in any operating solution the nitrate concentration should not exceed about 1.25 times the phosphate concentration and preferably is maintained somewhat below the phosphate concentration. As the ratio of No /P increases, the weight of coating obtained decreases and the corrosion resistance decreases, and

when the ratio of NO /PO, exceeds about 1.25, the significant increase in corrosion resistance which characterizes this invention is not obtained.

The process of this invention is especially suitable for forming corrosion resistant coatings on ferrous surfaces of particularly iron and steel. The resulting coatings have an average weight in the range of about 1,500-4,000 milligrams per square foot and may be subsequently provided with an oil, wax stain or even a paint finish, if desired. Any of the commercially available oils, Waxes or stains are satisfactory for use in this final treatment step.

The solutions are preferably ope-rated at or near their boiling point, for example in the range of about 190 F.- 210 F. and the parts to be coated may be immersed in the solution for -60 minutes, and preferably -40 minutes. It is undesirable to allow the solutions to boil since any sludge which may be present in the tank is thereby agitated and may be deposited on the surface being coated.

After the coating is formed on the surface, the coating is preferably treated with a hot dilute aqueous chromic acid containing solution for 30-45 seconds. The chromic acid solution may be any of the commercially available chromic acid solutions containing from about .01 to .5% CrO and may be an admixture with other acids such as phosphoric or the like. After the final rinse, the article may be dried, for example, in air, by centrifuging, by air blow-off or in a drying oven at temperatures up to about 325 F. and preferably below about 225 F. The dried article, without further treatment, represents one form of the improved product of this invention and for certain applications is sufficiently resistant to corrosion to satisfy the requirements, or the article may be treated with oil, stain or waxes by immersion in drying, semi-drying or non-drying types of oils which may optionally include rust inhibitors and/or dyes or coloring components, or the surface may be treated with a wax emulsion, or be painted, if desired. With water-displacing oils, the coated parts may he oiled immediately after Withdrawal from the final rinse without drying the surface. After the application of oil, any excess can be removed by centrifuging or drying in air or an oven to thus produce another form of the improved products of this invention.

The below given examples are intended to further illustrate the inventive principles of the invention and it is to be understood that the specific materials, concentrations thereof and conditions employed in their use are illustrative only.

Examples I-XVI encompass three independent tests, each of which includes its own control. Test I encompasses Examples I, II and III with Example I constituting the control. Test II includes Examples IV-IX inclusive, with Example IV constituting the control. Test III includes Examples X-XVI inclusive with Example X constituting the control.

Because of specific differences between the steel being treated, the compositions being employed, possible variations in test ratings assigned by different individuals inspecting the corrosion and making the ratings, the separate tests are not entirely correlative and conclusions with respect to corrosion improvement are most accurately made relative to the control in a single test.

Example I A zinc phosphate solution was prepared containing 0.6% zinc, 1.68% PO, and 0.58% N0 The bath was heated to 200 F. and was aged by processing a number of 4" x 6" cold rolled steel panels through the bath until the bath contained the ferrous ion in a concentration of .2-.38. The bath had a total acid of 27.2-29.4 points determined on a 10 ml. sample using 0.1 NaOH solution to a phenolphthalein end point and a free acid of 6.4-6.8 for a 10 ml. sample using 0.1 NaOH solution to a bromphenol blue end point.

A number of 4" x 6" cold rolled steel panels were conventionally cleaned and immersed in the bath for 30 minutes, removed, rinsed in cold Water for 1 minute and then immersed in a dilute aqueous CrO solution containing 0.05% C and having a free acid of 0.1-0.2 as determined on a 25 ml. sample using 0.1 NaOH to a brom-cresol green end point. The panels were removed and after drying were tested in 5% salt spray for corrosion resistance in accordance with the method of ASTM B 117-61 and rated in accordance with ASTM 1654-61, under which rating the degree of corrosion is indicated numerically from 10-0, with 10 representing no failure and 0 representing over 75% surface corrosion.

After 16 hours of exposure to the salt spray, the panels were inspected and were rated at 7.8, the intensity of the rust varying from light to heavy.

Example [I A portion of the zinc phosphate bath described above in Example I, prior to aging, was modified by the addition of citric acid to provide a 1% citric acid concentration. The solution was then aged by processing 4 x 6" cold rolled steel panels through the bath until the bath had a Fe++ concentration of .2-.44%, a total acid of 44.2-46.9 and a free acid of 15.9-16.7. A series of 4 x 6" panels were processed in this solution, at 200 F. by immersion for 30 minutes, Withdrawn, cold water rinsed, chromic acid rinsed and dried in accordance with the procedure set forth above in Example I. The panels were then subjected to the same salt spray corrosion test. After 16 hours exposure, the average rating of a number of panels was 9.7, with the intensity of the rust varying from very light to light.

Example IIII A portion of the zinc phosphate solution described above in Example I, prior to aging, was modified to contain 1% citric acid and 0.004% nickel, added as nickel nitrate Ni(NO -6H O. After aging, the bath contained 0.22%-0.46% Fe++, and had a total acidof 40.8- 44.2 and a free acid of 14.4-16.0. The plurality of panels treated in this bath by immersion at 220 F., after processing in CrO and subjecting to the same salt spray corrosion test, had an average rating after 16 hours of 9.9, with the intensity of rust varying from no rust to very light rust.

Example IV A portion of the zinc phosphate bath described above in Example 1, prior to aging, was modified to contain 1% citric acid. The bath was aged and after aging contained 0.22%-0.33% Fe++ and had a total acid of 43- 44.6 and a free acid of 14.4-16.2

A series of panels coated in this solution under the same conditions specified above in Example I and tested i1; 2?; salt spray after 14 hours had an average rating 0 Example V A portion of the zinc phosphate bath described above in Example I, prior to aging, was modified to contain 1% citric acid and 0.004% cobalt added as cobalt nitrate,

Co(NO -6H O. The bath was aged and after aging contained 0.22%-0.36% Fe++ and had a total acid of 45.2-47.2 and a free acid of 16.0-16.4.

A series of panels coated in this solution under the same conditions specified above in Example I and tested in 5% salt spray after 14 hours had an average rating of 8.1 with the intensity of the rust varying from no rust to very light rust.

Example VI A portion of the zinc phosphate bath identified above in Example I was modified to contain 1% citric acid and 0.00375% Mn, added as Mn(NO After aging, the hath contained 0.22-0.36 Fe++ and had a total acid of 45.0-48.0 and a free acid of 15.4-16.6.

A series of cold rolled steel panels coated in this bath in an identical manner to that specified above in Example I, after subjected to 14 hours of 5% salt spray, were found to have an average rating of 7.4 with the intensity of rust varying from no rust to light rust.

Example VII A portion of the zinc phosphate bath specified above in Example I was modified to contain 1% citric acid and 0.00142% Bi added as Bi(NO -5H O, which concentration is the molar equivalent of 0.004% Ni.

After aging, the bath contained 0.22%0.35% Fe++ and had a total acid of 45 17.6 and a free acid of 15.6- 16.4.

A series of cold rolled steel panels coated in the bath for 30 minutes at 200 F., CrO rinsed in accordance with the procedure in Example I, after 14 hours of 5% salt spray exposure, had an average rating of 8.0, with the intensity of the rust varying from no rust to very light rust.

Example VIII A portion of the zinc phosphate bath specified above in Example I was modified to contain 1% citric acid and 0.00047% lithium, added as LiCO which is the equivalent of 0.004% Ni. After aging, the bath contained 0.23-0.36% Fe++, and had a total acid of 44.2-47.6- and a free acid of 15.6-16.6.

A series of cold rolled steel panels coated in the bath for 30 minutes at 200 F., CrO rinsed in accordance with the procedure in Example I, after 14 hours of 5% salt spray exposure, had an average rating of 7.8, with the in tensity of rust varying from no rust to light rust.

Example IX A portion of the zinc phosphate bath specified above in Example I was modified to contain 1% citric acid and 0.0094% Ce added as ceric ammonium nitrate, which is the molar equivalent of 0.004% Ni.

After aging, the bath contained 0.19%-.36% Fe++, and had a total acid'of 43.2-46.6 and a free acid of 15.4-16.2.

A series of cold rolled steel panels coated in the bath for 30 minutes at 200 F., CrO rinsed in accordance with the procedure in Example I, after 14 hours of 5% salt spray exposure, had an average rating of 8.0 with the intensity of the rust varying from no rust to very light rust.

Example X A zinc phosphate solution was prepared containing 0.52% zinc, 1.64% P0 and 1.34% N0 The bath was heated to 200 F. and was aged by processing a numof 4" x 6" cold rolled steel panels through the bath until the bath contained the ferrous ion in a concentration of 0.07-4%-0.28%. The bat-h had a total acid of 32-36 points determined on a ml. sample using 0.1 NaOH solution to a phenolp-hthalein end point and a free acid of 7.6-8.8 for a 10 ml. sample using 0.1 NaOH solution to a bromphenol blue end point.

A number of 4" x 6" cold rolled steel panels were conventionally cleaned and immersed in the bath for 30 minutes, removed, rinsed in cold water for 1 minute and then immersed in a dilute aqueous CrO solution containting 0.05% CrO and having a free acid of 0.1-0.2 as determined on a 25 ml. sample using 0.1 NaOH to a bromcresol green end point. The panels were removed and after drying were tested in 5% salt spray for corrosion resistance in accordance with the method of ASTM B117-61 and rated in accordance with ASTM 1654-61, under which rating the degree of corrosion is indicated numerically from 10-0, with 10 representing no failure and 0 representing over 75% surface corrosion.

After 16 hours of exposure to the salt spray, the panels were inspected and were rated at 5.7, the intensity of the rust varying from light to heavy.

Example XI A portion of the bath identified above in Example I, prior to aging, was modified to contain 1% tartaric acid and 0.004% nickel. After aging, the bath contained 0.2'6%-0.42% Fe++ and had a total acid of 31.6-37.4 and a free acid of 70-100.

A series of cold rolled steel panels coated in the bath for 30 minutes at 200 F., CrO rinsed in accordance with the procedure in Example I, after 24 hours in the salt spray had an average rating of 7.5 with the intensity of rust varying from no rust to very light rust.

Example XII A portion of the bath identified above in Example I, prior to aging, was modified to contain 1% m-alonic acid and 0.004% Ni. After aging, the bath contained 0.17% Fe++ and had a total acid of 45.4 and a free acid of 11.8.

A series of cold rolled steel panels coated in the bath for 30 minutes at 200 F., CrO rinsed in accordance with the procedure in Example I, after 24 hours in the salt spray had an aver-age rating of 8.8 with the intensity of the rust varying from no rust to very light rust.

Example XIII A portion of the bath identified above in Example I, prior to aging, was modified to contain 1% malic acid and 0.004% Ni. After aging, the bath had a total acid of 42-430 and a free acid of 12.8, the ferrous ion not being established because of the presence of organic material.

A series of cold rolled steel panels coated in the bath for 30 minutes at 200 F., CrO rinsed in accordance with the procedure in Example I, after 24 hours in the salt spray had an average rating of 6.2 with the intensity of rust varying from very light rust to light rust.

Example XIV A portion of the bath identified above in Example I, prior to aging, was modified to contain 1% glutamic acid and 0.004% Ni. After aging, the bath contained 0.186 tFe++ and had a total acid of 17.4 and a free acid of 10.8.

A series of cold rolled steel panels coated in the bath for 30 minutes at 200 F., CrO rinsed in accordance with the procedure in Example I, after 24 hours in the salt spray had an average rating of 7.7 with the intensity of rust varying from very light rust to light rust.

Example XV A portion of the bath identified above in Example I, prior to aging, was modified to contain 1% aspartic acid and 0.004% Ni. After aging, the bath contained .37-.52 Fe++ and had a total acid of 37.2 and a free acid of 10.8.

A series of cold rolled steel panels coated in the bath for 30 minutes at 200 F., C-rO' rinsed in accordance with the procedure in Example I, after 24 hours in the salt spray had an average rating of 7.5.

Example XVI A portion of the bath identified above in Example 1,

prior to aging, was modified to contain 1% oxalic acid 7 and 0.004% Ni. After aging, thee ath contained 35.4% Fe++ and had a total acid of 35.038 and a free acid of 6.87.6.

A series of cold rolled steel panels coated in the bath for 30 minutes at 200 F., CrO rinsed in accordance with the procedure in Example I, after 16 hours in the salt spray had an average rating of 920 with the intensity of the rust varying from no rust to very light rust;

What is claimed is:

1. A method for forming zinc phosphate coatings on ferrous surfaces which comprises the steps of immersing the surface in a solution consisting essentially of 0.1%2% zinc, 0.8%-4% P 0.1%1% iron, and from about 0.125% to about 2% of at least one saturated, aliphatic polycarboxylic acid having 2-6 carbon atoms, the carbon atom adjacent to at least one carboxy group therein being substituted with a radical selected from the group consisting of hydroxy, amino and carboxy radicals, and maint aining said surface in said solution until a corrosion resistant coating is formed on said surface.

2. A method for forming zinc phosphate coatings on ferrous surfaces which comprises the steps of immersing the surface in a solution consisting essentially of 0.l%2% zinc, 0.'8%4% P0 0.1%1% iron, up to N0 and from about 0.125% to about 2% of at least one saturated, aliphatic polycarboxylic acid having 2-6 carbon atoms, the carbon atom adjacent to at least one carboxy group therein being substituted with a radical selected from the group consisting of hydroxy, amino and carboxy radicals, the ratio of N0 to P0 being less than 1.25, and maintaining said surface in said solution until a corrosion resistant coating is formed on said surface.

3. A method for forming zinc phosphate coatings on [ferrous surfaces which comprises the steps of immersing the surface in a solution consisting essentially of about .l% to about 2% zinc, from about .8% to about 4% P0 from about .l% to about 1% iron, up to about 5% nitrate, from about .125 to about 2% of at least one saturated, aliphatic polycarboxylic acid having 2-6 caribon atoms, the carbon atomadjacent to at least one carboxy group therein being substituted with a radical selected from the group consisting of hydroxy, amino and carboxy radicals, and up to about 0.05% of a metal ion selected from the group consisting of nickel, cobalt, lithium, bismuth, cerium and manganese, the ratio of N0 to P0 being in the range of about 0.5 to about 0.8, and maintaining said surface in said solution until a corrosion resistant coating is for-med on said surface.

4. A method for forming zinc phosphate coatings on ferrous surfaces which comprises the steps of immersing the surface in a solution consisting essentially of about .1% to about .7% zinc, from about 1% to about 2% P0 from about .2% to about .45 iron, from about .5% to about 1.6% nitrate, from about 0.5% to about 1.5% of at least one saturated, aliphatic polycarboxylic acid having 26 carbon atoms, the carbon atom adjacent to at least one carboxy group there-in being substituted with a radical selected from the group consisting of hydroxy, amino and carboxy radicals, and up to about 0.004%-.0=06% of a metal ion selected from the group consisting of nickel, cobalt, lithium, bismuth, cerium and manganese, the ratio of N0 to P0 being in the range of about 0.5 to about 0.8, and maintaining said surface in said solution until a corrosion resistant coating is formed on said surface.

5. A method as claimed in claim 4 wherein said metal ion is nickel.

6. A method as claimed in claim 4 wherein said polycarboxylic acid is citric acid and said metal ion is nickel.

7. A method for forming zinc phosphate coatings on ferrous surfaces which comprises the steps of immersing the surface in a solution consisting essentially of 0.1 %-2% zinc, 0.8%-4% (P0 0.l-1% iron, and from about 0.125 to about 2% of at least one saturate-d, aliphatic polycanboxylic acid selected from the group consisting of citric acid, tartaric acid, malonic acid, m-alic acid, oxalic acid, glutamic acid, and aspartic acid, and maintaining said surface in said solution until a corrosion resistant coating is formed on said surface.

8. A method for forming zinc phosphate coatings on ferrous surfaces which comprises the steps of immersing the surface in a solution consisting essentially of 0.1 %2% zinc, 0.8%4% P0 0.1%1% iron, up to 5% N0 and from about 0.125% to about 2% of at least one saturated, aliphatic polycarboxylic acid selected from the group consisting of citric acid, tartaric acid, malonic acid, malic acid, oxalic acid, gultamic acid, and aspartic acid, and up to about 0.05% of a metallic ion selected from the group consisting of nickel, cobalt, lithium, bismuth, cerium and manganese, and maintaining said surface in said solution until a corrosion resistant coating is formed on said surface.

9. A method in accordance with claim 7 wherein said acid is citric acid.

.10. A method in accordance with claim 8 wherein said acid is citric acid and said metal ion is nickel.

References Cited by the Examiner UNITED STATES PATENTS 1,493,012 5/1924 Abraham 1486.15 X

FOREIGN PATENTS 223,793 9/ 1959 Australia.

738,004 10/1955 Great Britain.

MURRAY KATZ, Primary Examiner.

RICHARD D. NEVIUS, R;. S. KENDALL,

Assistant Examiners. 

1. A METHOD FOR FORMING ZINC PHOSPHATE COATINGS ON FERROUS SURFACES WHICH COMPRISES THE STEPS OF IMMERSNG THE SURFACE IN A SOLUTION CONSISTING ESSENTIALLY OF 0.1%-2% ZINC, 0.8%-4% PO4, 0.1%-1% IRON, AND FROM ABOUT 0.125% TO ABOUT 2% OF AT LEAST ONE SATURATED, ALIPHATIC POLYCARBOXYLIC ACID HAVING 2-6 CARBON ATOMS, THE CARBON ATOM ADJACENT TO AT LEAST ONE CARBOXY GROUP THEREIN BEING SUBSTITUTED WITH A RADICAL SELECTED FROM THE GROUP CONSISTING OF HYDROXY, AMINO AND CARBOXY RADICALS, AND MAINTAINING SAID SURFACE IN SAID SOLUTION UNTIL A CORROSION RESISTANT COATING IS FORMED ON SAID SURFACE. 